Ancient Mars may have had a carbon cycle ā a new study suggests the red planet may have once been warmer, wetter and more favorable for life
by Elisabeth M. Hausrath, Professor of Geoscience at the University of Nevada, Las Vegas
Mars, one of our closest planetary neighbors, has fascinated people for hundreds of years, partly because it is soĀ similar to Earth. It is about the same size, contains similar rocks and minerals, and is not too much farther out from the Sun.
Because Mars and Earth share so many features, scientists have long wondered whether Mars could have once harbored life. Today, Mars is very cold and dry, with little atmosphere and no liquid water on the surface ā traits that make it a hostile environment for life. But some observations suggest that ancient Mars may have been warmer, wetter and more favorable for life.
Even though scientists observing the surface of Mars conclude that it was once warmer than it is today, they havenāt been able to find much concrete evidence for what caused it to be warmer. But aĀ study my colleagues and I published in April 2025Ā indicates the presence of carbonate minerals on the planet, which could help solve this puzzle.
Carbonate minerals contain carbon dioxide, which, when present in the atmosphere, warms a planet. These minerals suggest that carbon dioxide could have previously existed in the atmosphere in larger quantities and provide exciting new clues about ancient Marsā environment.
As aĀ geochemist and astrobiologistĀ who has studied Mars for more than 15 years, I am fascinated by Marsā past and the idea that it could have been habitable.
Ancient carbon cycle on past Mars
Observations of Mars from orbiting satellites and rovers show river channels andĀ dry lakesĀ that suggest the Martian surface once had liquid water. And these instruments have spotted minerals on its surface that scientists can analyze to get an idea of what Mars may have been like in the past.
If ancient Mars had liquid water, it would have needed a much warmer climate than it has today. Warmer planets usually have thick atmospheres that trap heat. So, perhaps the Martian atmosphere used to be thicker and composed ofĀ heat-trapping carbon dioxide. If Mars did once have a thicker carbon dioxide-containing atmosphere, scientists predict that theyād be able to see traces of that atmospheric carbon dioxide on the surface of Mars today.
Today, Mars is very cold, with a thin atmosphere and dry climate. But in the ancient past, it may have been warmer and wetter, with a thicker heat-trapping atmosphere.Ā NASA/J. Bell - Cornell U./M. Wolff - SSI via AP, File
Gaseous carbon dioxide dissolves in water, a chemical process that can ultimately contribute to formation of solid minerals at and below the surface of a planet ā essentially removing the carbon dioxide from the atmosphere. Lots of scientists have previously tried to find carbonate minerals on the surface of Mars, and part of the excitement about a warmer, wetter early Mars is that it could have been a suitable environment forĀ ancient microbial life.
Finding carbonates on Mars
Previous searches for carbonates on Mars have turned up observations ofĀ carbonates in meteoritesĀ and at two craters on Mars:Ā Gusev craterĀ andĀ Jezero crater. But there wasnāt enough to explain a warmer past climate on Mars.
For the past few years, theĀ Mars Science Laboratory Curiosity roverĀ has been traversing a region calledĀ Gale crater. Here, the roverāsĀ chemistry and mineralogy instrumentĀ has discovered lots of theĀ iron-rich carbonate mineral siderite.
The Curiosity rover has detected carbonates on Marsā surface.Ā NASA
As my colleagues and I detail in ourĀ new study about these results, this carbonate mineral could contain some of the missing atmospheric carbon dioxide needed for a warmer, wetter early Mars.
The rover also found iron oxyhydroxide minerals that suggest some of these rocks later dissolved when they encountered water, releasing a portion of their carbon dioxide back into the atmosphere. Although it isĀ very thin, the modern Martian atmosphere is still composed mainly of carbon dioxide.
In other words, these new results provide evidence for an ancientĀ carbon cycleĀ on Mars. Carbon cycles are the processes that transfer carbon dioxide between different reservoirs ā such as rocks on the surface and gas in the atmosphere.
Potential habitats for past microbial life on Mars
Scientists generallyĀ consider an environment habitableĀ for microbial life if it contains liquid water; nutrients such as carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur and necessary trace elements; an energy source; and conditions that were not too harsh ā not too acidic, too salty or too hot, for example.
Since observations from Gale crater and other locations on Mars show that Mars likely had habitable conditions, could Mars thenĀ have hosted life? And if it did, how would researchers be able to tell?
Although microorganisms are too small for the human eye to detect, they can leave evidence of themselvesĀ preserved in rocks, sediments and soils. Organic molecules from within these microorganisms are sometimes preserved in rocks and sediments. And some microbes can form minerals or have cells that can form certain shapes. This type ofĀ evidence for past lifeĀ is called a biosignature.
Collecting Mars samples
If Mars has biosignatures on or near the surface, researchers want to know that they have the right tools to detect them.
So far, the rovers on Mars have found someĀ organic moleculesĀ andĀ chemical signaturesĀ that could haveĀ come from eitherĀ abiotic ā nonliving ā sources or past life.
However,Ā determining whether the planet used to host lifeĀ isnāt easy. Analyses run in Earthās laboratories could provide more clarity around where these signatures came from.
To that end, theĀ Mars 2020 Perseverance roverĀ has been collecting and sealing samples on Mars, with one cache placed on theĀ surface of MarsĀ and another cache remaining on the rover.
These caches include samples ofĀ rock,Ā soilĀ andĀ atmosphere. Their contents can tell researchers about many aspects of the history of Mars, including past volcanic activity, meteorite impacts, streams and lakes, wind and dust storms, and potential past Martian life. If these samples are brought to Earth, scientists couldĀ examine them hereĀ for signs of ancient life on another planet.
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